trust_anchors: respect timestamps in root-anchors.xml

We are not RFC 7958 compliant and support only XML with just root zone
TA. Full compliance would require either proper Lua XML parser or CMS parser
and both are hard to get packaged in Fedora and elsewhere.
Also timestamps related to TA validity are limited to UTC timezone
because cross-platform timezone parsing is hard.
(Mac OS libc does not have usable strptime(%z).)
Closes: #435